Thanks Henning! I suspected some kind of optimization but I didn't think ghci would do any. I guess it is just a subtle consequence of eta reduction. It was very confusing to me.

Cheers

George

On Fri, May 17, 2024 at 6:43 PM Henning Thielemann <lemming@henning-thielemann.de> wrote:

On Fri, 17 May 2024, George Colpitts wrote:

> After I load the following into ghci and call main I expect to see "norm" in the output 3 times as I have called it 3 times
> but it only shows once. Why?
>
> traceBug.hs
> ---------------
> {-# OPTIONS_GHC -Wall #-}
>
> import Debug.Trace (trace)
>
> norm :: [Int] -> [Int]
> norm  = trace ("norm") id
>
> main :: IO ()
> main =
>   do
>     print $ norm [2]
>     print $ norm [2]
>     print $ norm [3]
> ---------------------
>
> ghci session
>
> % ghci traceBug.hs
> Loaded package environment from /Users/gcolpitts/.ghc/x86_64-darwin-9.8.2/environments/default
> GHCi, version 9.8.2: https://www.haskell.org/ghc/  :? for help
> [1 of 2] Compiling Main             ( traceBug.hs, interpreted )
> Ok, one module loaded.
> ghci> main
> norm
> [2]
> [2]
> [3]


I think this is correct. 'norm' is evaluated only once, because it is the
same variable (for a function) in every call. It would be different, if
you would define:

   norm xs = trace "norm" xs

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